In dustless abrasive blasting, an abrasive is entrained in a pressurized liquid flow or gaseous-entrained liquid flow and is directed against the surface to be treated by a controllable nozzle. It is the intention of such apparatus to coat each particle of the abrasive with the liquid so that the abrasive is weighted by the liquid and falls safely to the ground after striking the surface to be blasted, generally obviating the requirement for the operator of the apparatus to wear breathing apparatus. The weighted abrasive also increases the efficiency of the blasting operation.
Typically, the liquid used in abrasive blasting apparatus is water or a water based blasting solution, such as a rust inhibiting solution but it will be appreciated that any suitable liquid could be used. Hence, whenever the term “water” is referred to herein and in the claims it should be understood that the term includes all suitable blasting liquids. Similarly, the pressurized gaseous streams used in blasting operations are typically pressurized air but again the term “air” when referred to herein and in the claims is to be understood as including any gas suitable for use in abrasive blasting operations.
Conventionally, a dry abrasive is entrained into a gaseous stream and liquid is then added to wet the abrasive prior to its egress from the nozzle. In this way, dust generation at the work area is reduced. Such an arrangement, however, requires a separate pump to be used to deliver the liquid into the abrasive and gaseous stream, which makes the cost of the blasting apparatus more expensive than is desirable and renders the blasting apparatus more prone to breakdown.
In WO 88/07916, an apparatus is described which overcomes the aforementioned problems by entraining separately contained liquid and an abrasive/liquid mixture in a flow of pressurized fluid whilst simultaneously applying the pressurized fluid to the receptacles containing the liquid and the abrasive/liquid medium in order to balance the fluid pressure between the receptacles themselves and the receptacles and the delivery line. This has the advantage of requiring only a single pressurized fluid source and therefore of reducing the cost of the apparatus.
The aforementioned apparatus was further refined in WO 00/51787 by the injection of a pressurized fluid into the lower portion of the receptacle directly into the contents, which is thereby agitated. A control means is then provided to control the pressure within the receptacle to ensure that during blasting the pressure of the pressurized fluid entering the vessel through the injection means is always maintained at a higher level than the pressure of the pressurized fluid directed into the vessel by the pressurizing means. This enabled fine abrasive particles in addition to conventional abrasive materials such as various sands and grit, to be used in the apparatus.
However, the aforementioned apparatus all suffer from the disadvantage that a considerable quantity of water or other blasting liquid is used during a blasting operation. This makes the operation messy as large quantities of liquid must be disposed of and the object of the cleaning operation is thoroughly wetted, which can be disadvantageous.
Other apparatus entrain an abrasive/liquid mixture contained in a pressure vessel directly in a flow of pressurized air, the vessel being pressurized by water from a mains supply rather than by pressurized air.
This significantly reduces the quantities of waste water which is produced. However, this apparatus also suffers from the disadvantage that the entrained abrasive particles tend not to be evenly dispersed within the pressurized blasting fluid but clumped together in groups or pockets so that the resulting jet of blasting fluid is not uniform in nature. This slows down a blast cleaning operation as the operator must ensure that the blasting jet is played over the surface to be cleaned for a sufficient length of time to compensate for the unevenness of the jet.
The object of the present invention is to overcome the aforementioned disadvantages by considerably increasing the quantity of abrasive entrained in the pressurized fluid. This has the effect of increasing the ratio of abrasive to blasting liquid content, thus reducing the large quantities of liquid waste which results from a blasting operation. Also, the blasting jet is made more uniform and smooth in nature.